1. Field of the Invention
The present invention relates to a novel olefin polymerization catalyst comprising a modified clay compound having an average particle diameter of less than 10 .mu.m, a transition metal compound, and an organoaluminum compound, and to a process for production of an olefin polymer employing the catalyst.
2. Description of the Related Art
Catalyst systems comprising a combination of a transition metal compound and an organoaluminum compound are known for polymerization of olefins to produce an olefin polymer. Kaminsky, et al. disclosed a catalyst comprising a metallocene and methylaluminoxane having high activity in production of a polymer of an olefin including propylene in JP-A-58-19309.
The above catalyst system has some disadvantages, although it has high activity for olefin polymerization. The catalyst system, which is soluble in the reaction system, is usually employed in solution polymerization systems, limiting the production process. Relatively expensive methylaluminoxane is necessary in a large amount for production of a polymer having industrially useful properties. Therefore, the catalyst system causes a high production cost and a larger amount of residual aluminum remaining in the produced polymer, disadvantageously.
JP-A-60-35006, and other documents disclose catalyst systems in which the aforementioned soluble catalyst is supported by an inorganic oxide carrier like silica. However, as the results of replication tests of the disclosed methods, the catalyst activity was not sufficiently high for the amount of the employed methylaluminoxane.
For improvement thereof, JP-A-4-8704, JP-A-4-11604, JP-A-4-213305, and so forth disclose gas-phase polymerization processes in which a catalyst system having been treated by preliminary polymerization with a small amount of methylaluminoxane is used to obtain a polymer having satisfactory particle properties with high polymerization activity. Even in these processes, the catalyst activity is not sufficient although the amount of the aluminoxane used is small. Therefore, higher activity of the catalyst system has been demanded.
JP-A-1-503788 describes production of ethylene/.alpha.-olefin copolymer with a transition metal compound-aluminoxane catalyst by a high-pressure high-temperature process. However, industrial use of such a catalyst in a large scale involves problems that the methylaluminoxane cannot readily be synthesized with reproducibility as mentioned above, and the ratio of the expensive methylaluminoxane to the transition metal has to be significantly high to achieve sufficient activity.
JP-A-5-295022 and JP-A-7-309907 disclose solid catalyst systems employing a clay mineral or the like. However, the polymerization activity thereof is not satisfactory.
JP-A-7-224106 discloses catalyst system employing a clay modified by a cationic compound as the cocatalyst. This catalyst system is not satisfactory in process suitability. For example, the solid catalyst makes difficult the control of the particle size of the polymer powder in slurry polymerization or gas phase polymerization, producing coarse polymer particles adversely affecting the polymer properties. Further, the solid catalyst is not suitable for a high-pressure process or a solution process in which the residence time is short.
After comprehensive investigation to solve the above problems, it was found by the inventors of the present invention that a catalyst comprising a specific clay compound having a specified particle diameter, a transition metal compound, and an organoaluminum compound has high activity in an olefin polymerization process. The present invention has been accomplished based on the above findings.